This invention relates to X-ray tomography and, more particularly, to an apparatus and method for imaging cross-sections of objects using transverse tomography techniques. The subject matter of this invention is related to subject matter disclosed in U.S. application Ser. No. 587,329, entitled "Apparatus For Obtaining One-Dimensional Projections" filed of even date herewith and assigned to the same assignee as the present application and now abandoned.
Using conventional X-ray imaging techniques, a shadow view of a body under examination can be produced, but it does not contain information concerning the depth of details in the body. In many cases it is not possible to distinguish small objects since they are obscured by the structures of larger objects; e.g., bones. This drawback has been overcome to some degree by the development of body-section radiography techniques known as X-ray tomography.
Radiographic transverse tomography attempts to view sections or "slices" which are perpendicular to the axis of a patient. In a "classical" transverse tomographic system, incident X-radiation is passed through an object being studied at an angle and is imaged on a film. In order to image a particular slice, both the film and the object are rotated synchronously during the X-ray exposure. Shadows of points in a single plane of the object are continually projected on to the same place on the film during the rotation, whereas shadows of other parts of the object move in relation to the film. Thus, while the slice of interest is imaged relatively sharply, the resultant picture is overlayed by the motion-blurred images of other parts of the object. As a consequence, the resultant "tomogram" tends to lack contrast and fine detail is obscured.
Recently, digital processing techniques have been applied to the tomographic imaging problem and a degree of success has been achieved whereby blurred images of overlying and underlying planes have been removed so as to permit detection of greater detail. However, digital methods require the processing of extremely large quantities of data, and even fast computers take significantly long times to do the necessary computations. Accordingly, commercial computer tomography equipment is extremely expensive and beyond the financial reach of many who desire it.
In another type of transaxial tomography, a narrow beam of X-rays is employed and only the rays passing through the desired cross-section are incident on a film so, ideally, only information about the particular slice is recorded. The result is a so-called "one-dimensional projection". A plurality of one-dimensional projections can be obtained by passing X-rays through the same cross-section at a number of different rotational angles. The resultant set of one-dimensional projections can be processed optically. Alternatively, the values of each projection can be fed to a computer for digital analysis, whereby the density function of each elemental area in the plane is computed by one of a number of mathematical techniques which utilize iteration, mathematical filtering techniques, and other known solutions.
An early technique for optical processing of a set of one-dimensional projections is disclosed in U.S. Pat. No. 2,281,931 wherein a cylindrical lens system is utilized to optically "enlarge" each one-dimensional projection in a direction perpendicular to the plane of the section. Each enlarged one-dimensional projection is a two-dimensional image and the set of two-dimensional images which result from optically enlarging each one-dimensional projection are superposed with mutual angular displacements that correspond to the rotation angles at which the one-dimensional projections were originally taken. The image ultimately produced in this manner has been referred to as a "layergram" of the cross-section. In recent years, attempts have been made to process the layergram using spatial filtering methods of both optical and digital natures to restore the layergram image which is known to suffer blurring. However, the digital processing techniques again involve the handling and lengthy processing of large amounts of data, which is very expensive. Optical processing techniques toward this same end have generally been found to be either inadequate from a performance standpoint or unduly complex and expensive.
The techniques described in the above-referenced U.S. Pat. No. 2,281,931 is one of a number of image reconstruction techniques which utilize the "back-projection" of an image. Generally, the term "back-projection" infers that the value of a particular point in a projection is assigned to all points on a line perpendicular to the projection. The values of overlapping lines are integrated for all projections. The result is equivalent to back-projecting the values in each one-dimensional projection through the object and integrating their overall effect. As implied above, a simple back-projection yields results which are generally considered inadequate, and it is presently believed that a technique of back-projection, combined with a suitable filtering technique, could yield quality results. However, as emphasized above, such techniques have in the past required expensive and complex systems.
It is one object of the present invention to provide an improved relatively inexpensive apparatus for generating a filtered two-dimensional back-projected image of a plane or slice of an object by processing a set of one-dimensional projections of the object. It is a further object of the invention to provide an improved technique for initially obtaining the one-dimensional projections.